A G Scale train model compared to a finger. Shown at the Chicago Botanic Garden.
Photo: (released into the public domain)

U.K. Prototype model of an OO scale (1:76.2) en:British Railways en:Class 25
shown with 5 en:pence for size.Photo by William J. Grimes. (released into the public domain)

HO Scale GE 44-ton switcher made by Bachmann, shown with a pencil for size.
author: Brakeman Billy (released into the public domain).

Photo by: Emil Kindlein. Used by permission.

Photo of a Bachmann 2-6-6-2 N scale steam locomotive with a pencil shown for size
Photo:  released into the public domain.

Photo:  released into the public domain.

Photo of a section of an N scale layout with a human hand for size comparison
Author: Freddie10538 (released into the public domain)

The source for all pictures was www.wikipedia.org except were noticed.